1. Technical Field
This invention relates to constant current sources for power amplifiers, and more particularly, to a current mirror biasing circuit that provides a constant quiescent collector current for a power transistor over a varying collector supply voltage.
2. Background Information
Current mirror biasing circuits allow for constant DC quiescent collector currents in power transistors. The DC quiescent collector current is the DC current flow into the collector of the transistor with no radio frequency (RF) signal applied to the base of the transistor. FIG. 1 illustrates a known current bias circuit 100 used in a mobile electronic device such as a cellular telephone (cell phone). As shown, the circuit contains a current mirror transistor Q1 a current mirror bias transistor Q3, an RF power amplifier transistor Q2, and a power amplifier bias transistor Q4. The emitter 28 of current mirror transistor Q1 is grounded and the collector 26 of current mirror transistor Q1 is connected to a power terminal 10 through a constant current source 7. The power terminal 10 is a terminal to which a power supply (not shown) is connected. The emitter 25 of the current mirror bias transistor Q3 is connected to the base 27 of the current mirror transistor Q1 through a base ballast resistor 15. The collector 24 of current mirror bias transistor Q3 is directly connected to the power terminal 10. The base 23 of current mirror bias transistor Q3 is connected to a connection point between the collector 26 of current mirror transistor Q1 and the constant current source 7. Also connected to this same connection point is the base 13 of the power amplifier bias transistor Q4. The collector 14 of the power amplifier bias transistor is directly connected to the power terminal 10, and its emitter 17 is connected to the base 5 of RF power amplifier transistor Q2 through a base resistor 16. The connection point between the base resistor 16 and the base 5 of the RF power amplifier transistor Q2 is connected to an RF input terminal 11. The collector 18 of the RF power amplifier transistor Q2 is connected to the power terminal 10 through load 8, and the emitter 19 is grounded. The output of the circuit is taken from a connection point 12 between the collector 18 of RF power amplifier transistor Q2 and load 8.
The amplification of an RF signal occurs at the RF power amplifier transistor Q2. The RF input signal is applied to the RF power transistor Q2 at an input terminal 11, and the RF output is available at the output terminal 12. A bias-voltage for the RF power transistor is generated by the circuit that includes a constant current source 7, a current mirror transistor Q1, a current mirror bias transistor Q3 and a base ballast resistor 15. The voltage developed at the collector 26 of the current mirror transistor Q1 biases the power amplifier bias transistor Q4 which in turn biases the RF power transistor Q2 through a base resistor 16. Power VCC to the overall circuit is supplied at terminal 10.
In the known art, the voltage drops across each of the mirroring elements should equal its counterpart. In other words, the base-emitter voltages VBE2 and VBE1 for the RF power transistor Q2 and the current mirror transistor Q1 will be equal; the voltage drop, VR15 and VR16, across resistor 15 and resistor 16 will be equal; and the base-emitter voltages, VBE3 and VBE4 for the current mirror bias transistor Q3 and the RF power transistor Q4, respectively, will be equal. Since the transistors are tied to the power terminal 10 in this embodiment, should the voltage at the power terminal 10 decrease, then the quiescent collector currents for the transistors will decrease as the bias currents decrease.
The quiescent collector current is linked to the power added efficiency (PAE) of the amplifier, the output power (POUT), the gain, the optimal output impedance match, and the reliability of the device. The power added efficiency is defined as the output power (the RF power at the fundamental frequency supplied at the output terminal 12) minus the input RF power at the fundamental frequency, divided by the DC power of the entire circuit. The RF power transistor Q2 should be biased below a certain collector current density to extend the lifetime of the device and to avoid damage to the transistor. The collector current density is the current through the collector divided by the area of the collector (i.e. current/unit area). Changes in the load 8 or the biasing conditions of the power transistor Q2 can alter the value of the impedance match present at the output terminal 12. This alteration in the impedance match causes the quiescent collector current density to rise to a level where the power transistor Q2 sustains catastrophic damage.
Although the quiescent collector current may remain constant as the collector supply voltage varies, the output power of the amplifier can vary with the voltage. Even if the voltage did not vary, the power of the transmitting device may need to be increased or decreased depending upon the application. For instance, where a mobile cell phone is continually moving and changing its distance from the base station, the power to maintain communication with the base station may need to increase as the distance increases between the base station and the cell phone. On the other hand, when the cell phone is closer to the base station, the output power may be reduced for the shorter-range communication. Therefore, there is a need for a current mirror bias circuit with the capability of providing an approximately constant quiescent collector current in the RF power transistor Q2 despite a changing voltage supplied to the power terminal 10, but yet provide a varying reference voltage to prompt changes in the collector current of the power transistor Q2 to meet the varying power requirements of the amplifier.